1. Field of the Invention
The present invention relates to a switchable d.c. voltage regulation circuit.
2. Discussion of the Related Art
Such a circuit is schematically shown in FIG. 1 and is referred to with reference 1. It is connected by its input to a d.c. voltage Vin and outputs a voltage Vout which must remain as constant as possible when Vin varies or when the current Iout in a load L varies. This circuit is provided with a control input CTRL to output either voltage Vout or a zero voltage. An application of such a circuit is, in the automobile field, to supply a light-emitting diode or a chain of light-emitting diodes. Such light-emitting diodes can, for example, be used as the third tail light of a car. Thus, voltage Vin is the battery voltage of the vehicle and can vary significantly.
In the following, it will be assumed for simplification that voltage Vin and voltage Vout are positive voltages referenced to the ground.
FIG. 2 shows an elementary voltage regulation circuit. The voltage regulation is performed by an avalanche diode Z, the anode of which is grounded and the cathode of which is connected on the one hand to a regulated output terminal Vout and on the other hand to an input terminal Vin via a resistor R1. A switch such as a transistor TR1 is arranged between terminal Vout and the ground. The base of this transistor receives control voltage CTRL. Thus, when the transistor is off, a voltage Vout substantially equal to the avalanche voltage of avalanche diode Z is present at the output. This circuit has several drawbacks. A first drawback is the presence of power resistor R1. If, for example, output voltage Vout has to be regulated to 10 V and voltage Vin rises to a value of 30 V, the voltage drop across the resistor will be on the order of 20 V and for a resistance of 50 ohms, a dissipated power of 1 Watt is reached. Such power resistors are expensive. Another drawback of the circuit of FIG. 2 is that the current in avalanche diode Z is likely to greatly vary when voltage Vin varies. As a result, the output voltage variation can be significant.
Another series resistor assembly is illustrated in FIG. 3. A resistor R1 is connected between terminals Vin and Vout as in FIG. 2. An avalanche diode Z is connected between the collector and the base of transistor TR1, itself connected between Vout and the ground. A biasing resistor R2 is connected between the base and the emitter of transistor TR1. In this case, the nominal regulation voltage is the voltage of the avalanche diode plus the base/emitter voltage of transistor TR1. The same drawback of use of a series resistor in the main current circuit appears in this assembly. An advantage with respect to the assembly of FIG. 2 is that voltage Vout varies less with the variations of voltage Vin.
To avoid the drawbacks of circuits with a series resistor, circuits in which a semiconductor component, generally less expensive than a power resistor, is arranged in the branch in series between input terminal Vin and output terminal Vout have also been provided in prior art. This semiconductor component further enables to interrupt the current in the power branch and thus to limit losses during phases where a zero output voltage is desired.
FIG. 4 shows an example of a circuit with a gate turn-off (GTO) thyristor. A GTO thyristor Th1 is connected by its anode to terminal Vin and by its cathode to terminal Vout. A resistor R3 is connected between the anode gate and the cathode gate. The cathode gate is grounded via an avalanche diode Z and possibly a forward-biased diode d to perform a temperature compensation function. A transistor TR2 is connected between the cathode gate of thyristor Th1 and the ground. The base of transistor TR2 is connected to a control terminal CTRL. When the transistor is off, the thyristor is normally on under the effect of its gate biasing due to resistor R3. Output voltage Vout is regulated to the cathode/gate voltage drop plus the voltage of avalanche diode Z. When transistor TR2 is turned on, the thyristor turns off and voltage Vout becomes substantially zero. The anode gate could also not be used, and resistor R3 could be directly connected to the thyristor anode. The assembly shown has the advantage of ensuring protection in case of an inversion of the biasing of voltage Vin, which can occur when the voltage source corresponds to an automobile battery.
Another circuit with a semiconductor component is shown in FIG. 5. Thyristor Th1 is replaced with a transistor TR3. The other circuit elements are similar to those of FIG. 4. This circuit notably has the disadvantage of requiring a transistor with a relatively high gain, which is relatively difficult to obtain in the case of a power transistor with a high direct breakdown voltage.